Materials have a quality that produces a very useful result, called resonance, and it is one of the things that gives added effect to a speaker’s voice in a hall, where there is a constant succession of echoes.
A wall distant from the speaker about 55 feet, produces an almost instantaneous reflection of the sound, and at double that measurement the effect is still stronger. When the distance is too short for the reflecting sound to be heard, we have resonance. It enriches the sound of the voice, and gives a finer quality to musical instruments.
Title: Practical Mechanics for Boys
Author: J. S. Zerbe